The direct current (DC) communication power supply system is divided into the positive system and the negative system by different working grounds, the working ground in the positive system is a negative row (V−), while the working ground in the negative system is a positive row (V+); at present, generally all the 24V DC communication power supply systems are the positive systems, and all the 48V DC communication power supply system are mainly the negative systems.
In the DC communication power supply system, for the load output, the load overcurrent protector needs to be configured to perform the overcurrent or short circuit protection, and the choice for the load overcurrent protective devices are often the air switch, fuse wire, etc.; the state of the load overcurrent protector is critical for the maintenance of the communication power supply system, and is the essential detection content in the power supply system monitoring. When the state of the load overcurrent protector is detected, it needs to distinguish three kinds of states: 1. the conducting state of the load overcurrent protector, which is the normal running state; 2. the disconnected state of the load overcurrent protector with the load, which is an abnormal state; 3. the disconnected state of the load overcurrent protector without the load, which is a normal state.
Generally, the detection approaches of the state of the load overcurrent protector are mainly as follows: 1. the detection is performed by using the assisted dry contact; when the overcurrent protector is disconnected, the state of the assisted dry contact changes; detecting the state of the assisted dry contact refers to detecting the state of the current overcurrent protector. This approach can isolate the detection loop with the loop of the main load, and can be compatible with the positive and negative system at the same time; but this approach is unable to distinguish the disconnected state of the overcurrent protector without the load; and it needs the software to cooperate to prevent the false alarm. 2. when the fuse wire is disconnected, the detection circuit of the overcurrent protector is used to form a circuit loop, and the abnormal state (the disconnected state of the overcurrent protector with the load) is distinguished from other states to perform the detection. Generally, it is very difficult for this approach to be compatible with the positive and negative system.